Vertical water entry of a long cylindrical rod with zero inclined angle: crater dynamics and droplet generation

The vertical impact of a long cylindrical rod with zero inclined angle (parallel to the water surface) on a quiescent water surface is studied experimentally. We focus on examining the crater dynamics and the droplets produced only as a direct result of pinching off as the crater collapses. Three solid steel cylindrical rods are used in the current study, with the same length of 91.4 cm and different diameters of 0.635, 1.270 and 1.905 cm, respectively. Electromagnets are used to attach and release each rod from a fixed height of 15 cm above the water surface, corresponding to a free-fall entry speed of 1.72 m/s. A hydrophobic coating is applied on the surface of each rod to guarantee the formation of a relatively smooth 2D crater. The evolution of the crater profiles is measured by laser induced fluorescence (LIF) imaging technique and the retraction speed of the crater tip formed right after pinch-off occurs is measured using back-lit diffuse light movies. The droplets produced are measured with an in-line cinematic holographic system. The positions, diameters (d ≥ 100 μm), times and velocities of droplets are measured as they move up across a prescribed horizontal measurement plane. Preliminary results show that as the rod diameter increases, the depth and width of the crater profile at pinch-off moment and the retraction speed of the pinch-off tip increase, which seem to be responsible for the observed increased number of produced droplets. The distributions of droplet size and velocity components are also presented.